This invention relates to systems and devices for connecting external auxiliary electrical power to an internal aircraft power system, and particularly to the electrical power-on control circuits used to start power flowing to an aircraft.
Commercial aircraft need to be supplied with electrical power while parked at an airport gate, prior to departure.
This electrical power is supplied either by a mobile power unit or by an equipment electrical power installation at the airport, providing auxiliary power. In either case, the power, which is usually 3-phase 400 Hz ac, is supplied through a cable having a large male connector plug. The installation or mobile power connector plug is first pushed in until it is properly mated with the aircraft power connector socket. The external power source is then manually switched on and power flows through the power connector into the aircraft.
A power-on control circuit, which may be located anywhere convenient, is used to start a mobile or installation power source, or simply to close contactors connecting power from an operating source. This power-on control circuit is connected to terminal pins in the external power connector plug used to make connection with an aircraft power system.
When the external power connector plug is plugged in the aircraft socket, the power-on control circuit senses whether a dc signal from the aircraft is present. If it is not present after a fixed time period, the external power source is automatically shut down. This action is taken to avoid continuing to supply power to an aircraft if a power connection is not properly made. A service technician can then go and correct the power connector condition.
In the presently available power-on control circuits employed in aircraft external power source connectors, an external power source is immediately activated by a push-button or switch and remains activated for at least four seconds. During this four second period, the connector plug is live with high power whether or not it is connected to the aircraft. This can be a cause of damage to the aircraft internal power supply if any connection pin is open, particularly if the connector neutral is open. On some occasions, it has been reported that an external power connector plug either fell out of the aircraft socket on to the ground or was not put in place at all. If the connector plug should land in a puddle of fluid, an external source start-up could be hazardous indeed.
While the presently employed external power control circuits for the most part operate satisfactorily, occasionally there are circuit failures caused by received power spikes or coupled signal pickup. Thus an isolation of power-on control circuit switching elements is highly desirable.
It is primarily to avoid the potential damage and hazards inherent in the use of available power-on control circuits which connect external power to aircraft, that the present invention power-on control circuit is offered. A secondary objective is a reduction in power-on control circuit failures.